Final answer:
In the endosymbiotic theory, Bacteria is the lineage thought to be the source of mitochondria. This is supported by genetic similarities between mitochondrial DNA and bacterial DNA.
Step-by-step explanation:
In Lynn Margulis's endosymbiotic theory proposed in 1970, the prokaryotic lineage that is a phylogenetic candidate as the source of mitochondria is Bacteria. Specifically, it is believed that mitochondria evolved from an aerobic bacterium that entered into a symbiotic relationship within a eukaryotic host cell. This theory has been supported by DNA sequencing and phylogenetic analysis indicating that mitochondrial DNA is highly related to bacterial DNA. Moreover, organelles like mitochondria and chloroplasts have similarities with bacteria, such as their own DNA, ribosomes like those of bacteria, and a method of division that resembles bacterial binary fission.